Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by x-ray diffraction. Such zeolites have pores of uniform size which are uniquely determined by the structure of the crystal. The zeolites are referred to as "molecular sieves" because the uniform pore size of the zeolite material allows it to selectively sorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
In the empirical formula, M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium; x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated M; and the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. D. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, N.Y., p.5 (1974).
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-11 (U.S. Pat. No. 3,709,979) and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The particular faujasitic or Y-type zeolite utilized in this invention has come to be known as ultrastable Y (USY) and is sometimes referred to as dealuminated Y (DAY). A partial list of references describing the nature and methods of preparation of USY or DAY, all of which are incorporated herein by reference are:
1. Maher, P. K., U.S. Pat. No. 3,293,192. PA1 2. Kerr, G. T., J. Phys. Chem., 71: 4155 (1967). PA1 3. McDaniel, C. V., U.S. Pat. No. 3,607,403. PA1 4. Maher, P. K., U.S. Pat. No. 3,402,996.
5. Scherzer, J., "The Preparation and Characterization of Aluminum Deficient Zeolites", ACS Symposium Series, Paper No. 10, June 13-16, 1983, pp. 157-200.
It is clear from these references, and other scientific and patent literature that USY is not a single entity but a family of materials related to zeolite Y. USY is similar to zeolite Y in that its characteristic x-ray diffraction lines are substantially those of zeolite Y as detailed in Tables A, B and C of the above referenced U.S. patent and herein incorporated. USY differs from as-synthesized zeolite Y in that by the nature of the various processing schemes and the degree to which zeolite Y is dealuminated, the effective framework silica-to-alumina ratio is increased. One measure of this change is reflected in the measurement of unit cell size of the resultant zeolite, usually reported in the atomic unit, Angstroms (A). As aluminum is removed from the zeolitic framework, hence causing the zeolitic framework silica-to-alumina ratio to increase, the unit cell size decreases. This results because of differences in bond distances between AlO.sub.4 tetrahedra and SiO.sub.4 tetrahedra.
U.S. Pat. No. 4,309,280 suggests the use of crystalline zeolites in hydrocarbon conversion processes. Specific processes relating to the cracking of gas oils to produce motor fuels have been described and claimed in many patents including, for example, U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253 and 3,271,418, herein incorporated by reference. In one or more of the above identified patents the combination of zeolites with a matrix for use in catalytic cracking is suggested.
Other references disclose the use of USY or DAY to crack alkanes. For example, A. Corma, et al., in APPLIED CATALYSIS, Vol. 12 (1984), pp. 105-116, present a "Comparison of the Activity Selectivity and Decay Properties of LaY and HY Ultrastable Zeolites During the Cracking of Alkanes". Pine, L. A., et al., in the JOURNAL OF CATALYSIS, Vol. 85 (1984), pp. 466-476 present data to support the "Prediction of Cracking Catalyst Behavior by a Zeolite Unit Cell Size Model". The performance of cracking catalysts containing USY or DAY are often compared to catalysts containing zeolite Y which has not been intentionally dealuminated. Because of the deleterious effect of sodium on the performance of cracking catalysts, USY or DAY catalysts are frequently compared with catalysts containing the hydrogen form of Y zeolite (HY) or the rare earth form of Y zeolite (REY).
In general the patent and scientific literature suggests the following for cracking catalysts containing USY or DAY, containing substantially no rare earth (Those claims being at constant conversion relative to REY containing cracking catalyst): 1. significant increases in gasoline research and motor octane (unleaded); 2. significant decreases in coke make; 3. definitive increases in total C.sub.3 +C.sub.4 make, particularly C.sub.3 olefins and C.sub.4 olefins; 4. reductions in gasoline yield.
Furthermore, lower catalytic activity is evidenced with decreasing unit cell size (U.C.S.) of the Y zeolite component. Hence a non-rare earth containing USY or DAY zeolite would exhibit lower activity/stability than a non-dealuminated REY zeolite because the former has a lower U.C.S. both as manufactured and subsequent to equilibration in a conventional cracking unit.
When rare earth components are introduced into these USY or DAY containing catalysts (RE-USY), irrespective of whether they are pre-exchanged onto the zeolite or post-exchanged onto the catalyst the increases in gasoline research and motor octane (unleaded), the increases in C.sub.3 and C.sub.4 production and the decreases in coke make are diminished in proportion to the amount of rare earth added. Furthermore lower catalytic activity for the RE-USY is still evidenced relative to non-dealuminated REY.
The catalyst of the present invention as disclosed below performs in a significantly different manner which was not a priori anticipated.